1. Field of the Invention
This invention relates to a method for preparation of a photosensor which is used for the taking out of light signals in a photoelectric transducing device for image information processing.
2. Description of the Prior Art
As the photoelectric transducing means constituting the image reading portion such as in a facsimile, a digital copying machine, etc., photosensors have been used as generally well known in the art. Such photosensors may include, for example, planar type photoconductive photosensors which are prepared by imparting a pair of metal electrode layers confronted so as to form a gap for the light receiving portion on a photoconductive layer comprising chalcogenide, CdS, CdS-Se, or amorphous silicon (hereinafter written as "a-Si"). Since a photoconductive photosensor can form readily a line sensor array in continuous length, studies have been made thereon particularly in recent years. Above all, the photosensor employing a-Si as the photoconductive layer is excellent particularly in light response and expected to be a photosensor capable of high speed reading.
FIG. 1 is a schematic sectional view showing one example of the method for preparation of a planar type photosensor of the prior art. On a glass substrate, for example, a glass substrate 11 made of #7059 produced by Corning Co., a photoconductive layer of a-Si (intrinsic layer) 12 is deposited by use of the plasma CVD (PCVD) method, A1 layer is formed uniformly all over the surface thereof and then A1 at the unnecessary portion is removed to form a sensor gap 14 for the photoelectric transducing portion, thus forming a pair of electrodes 15 and 16. FIG. 2 shows an example of the V-I characteristics during light irradiation in a photosensor with a sensor gap of 200.mu.. According to this Figure, it can be seen that, when the electric field intensity exceeds approximately 50 V/cm, the diode in opposite direction existing at the interface between the electrodes 15 and 16 and the photoconductive layer 12 becomes predominant, whereby no ohmic characteristic can be exhibited (non-ohmic region). At the same time, in this region, response of the photosensor to the turning-on or turning-off of light becomes very slow, and giving no high performance. Accordingly, it has been practiced to provide an n+ layer which is an ohmic contact layer interposed at the interface between the a-Si photoconductive layer 12 and the electrodes 15 and 16, thereby permitting the ohmic characteristic to be exhibited even under a high electrical field. FIG. 3 shows an example of the V-I characteristics during light irradiation of a photosensor having such an n.sup.+ layer.
Whereas, since a-Si is lower in heat resistance than crystalline silicon, it is impossible to use ion implantation or thermal diffusion as a means for doping for the n.sup.+ layer formation. Accordingly, the n+ layer which is an ohmic contact layer is generally formed according to the PCVD method by use of a starting gas for PVCD in which a doping gas such as PH.sub.3, AsH.sub.3, etc. is incorporated. Thus, the n.sup.+ layer is deposited uniformly all over the surface, and therefore the n.sup.+ layer at the portion corresponding to the sensor gas needs to be removed, and the n.sup.+ layer between the adjacent bits also needs to be removed when preparing a line sensor in continuous length. As a means for removing the n.sup.+ layer, there are the methods in which etching is effected with a mixture of hydrofluoric acid, nitric acid and acetic acid (the wet etching method) and the method in which etching is effected by plasma discharging of a gas composed mainly of a carbon halide (the plasma etching method). However, these etching methods known in the art involve drawbacks as described below.
(A) The drawbacks of the wet etching method:
(1) dangling bonds will increase at the etched surface to increase the dark current, which is the noise component;
(2) etch pits will be formed on the surface even by use of an etchant with an etching rate of 100 .ANG./sec;
(3) selectivity for the n.sup.+ layer and the photoconductive layer is too great with the above etchant, whereby the n.sup.- layer existing at the interface between the n.sup.+ layer and the photoconductive layer cannot be removed, which brings about an increase in dark current or an increase in variance of characteristics;
(4) the glass substrate surface is roughened by hydrofluoric acid contained in the etchant, and therefore the light quantity reaching the photoelectric converting portion is reduced in a photosensor of the form in which light is permitted to enter from the glass substrate side.
(B) The drawbacks of the plasma etching method:
(1) The surface subjected to etching has increased dangling bond, to increase the dark current, which is the noise component;
(2) Implantation of the cathode material such as SUS occurs.
Accordingly, only photosensors with low S/N ratio and great variance in performance could be prepared by use of either the etching method or the plasma etching method.